In the same way standard computers are most easily build out of bits, quantum computers are most often though of using quantum bits (qubits) as building blocks. These systems are discrete: they have exactly two perfectly distinguishable states, 0 and 1. For example, the spin of an electron along one direction has this structure: it can only be up or down.

Although other quantum systems have a richer structure. Take light, in a given mode there can be any number of photons. Even more if you measure the amplitudes of the electric and magnetic fields, the answer could be any real number. These systems are continuous variable quantum systems.

Using carefully crafted cavities we can trap and manipulate such continuous variable quantum systems. They can be then used to encode qubits in many different ways. Some ways even allow for some mechanism for error correction. They can also be used directly for some protocols using continuous variables.